This invention relates to hydraulic actuators, and is more particularly concerned with piston type actuators employed at high frequencies and where quick response is required. The invention is more specifically directed to actuators for opening and closing intake and/or exhaust valves for internal combustion engines.
It has been recently proposed to employ hydraulic valve actuators in diesel and gasoline engines to replace the conventional system of camshaft, cams and rocker arms.
In the hydraulically actuated system, hydraulic actuators are associated with each of the intake and exhaust valves. Each actuator fits into a respective socket or receptacle in the engine cylinder head, which is in the form of an aluminum block with chambers, bores, and passages for the hydraulic oil to flow to and from these actuators. A timing wheel turns synchronously with the engine crank shaft (once for every two crank turns in the case of a standard four-stroke engine). A sensor measures the wheel position and determines the crankshaft position, that is the phase of the various pistons as they oscillate in their respective cylinders or combustion chambers. The sensor is connected to a timing circuit that sends pulse width modulated (PWM) signals to each of the solenoid valves. The latter each open or close to send fluid pressure to the associated actuator and thus open or close it intake or exhaust valve in accordance with the piston phase.
With this system, it is possible to adjust valve timing on the fly by adjusting the timing and shape of the PWM signals from the timing circuit.
However, because of the sustained high speeds (operating rate of 50 Hz at an engine speed of 6000 RPM) and the precision needed for timing of the valves, certain problems arise in the construction and operation of the actuators.
Because the actuators must operate over a wide range of operating conditions in which the temperature can vary by 300 degrees F., but close tolerance must be maintained, all major parts of the actuator must be made of the same material (usually steel) so that all the elements have substantially the same coefficient of thermal expansion. However, if sliding contacting parts, i.e., the piston and the rod bearing, both are formed of the same metal then there is a significant risk of adhesive wear, i.e., galling.
The conventional approach to this problem of galling is to provide a coating of a soft metal, such as copper, on one or the other of the surfaces in sliding contact. This technique as applied to a high-performance spool valve is described in U.S. Pat. No. 4,337,797. Providing the opposing sliding surfaces with widely varying hardnesses avoids adhesive contact such as galling.
However, in applications where there is heavy-duty service and high-velocity motion of the sliding parts, damage occurs to the soft copper surfaces caused by localized cavitation. The cavitation results in erosion of the copper. The erosion exposes the underlying steel body of the unit and permits direct contact with the sliding spool or piston within. This brings about galling at high velocities.
Cavitation occurs when, because of quick piston movement, the fluid pressure in a localized area falls below the vapor pressure of the fluid. This can result in evaporation of the liquid film layer that normally is found between the piston and cylinder cavity and between the rod and the rod bearing. In the absence of this liquid film, direct metal contact can occur. Then normal piston motion can wear away the soft metal coating.
Because of this characteristic, the copper plated hydraulic actuator is poorly suited for use in applications such as with the intake or exhaust valve of an internal combustion engine where the piston must be driven at high velocities and at high frequency for extended periods.
Consequently, the industry has sought a hydraulic actuator which can be employed in this environment and which avoids the above-noted problems attributed to cavitation and galling.